1. Field of the Invention
The present invention relates to a single lens reflex camera having diaphragm mechanism automatically controlled upon shutter releasing in accordance with scene brightness measured through the camera objective and the diaphragm aperture.
2. Prior Art
In U.S. Pat. No. 3,777,637 a single lens reflex camera of the above mentioned type is described wherein the diaphragm in an interchangeable lens, is activated upon shutter release operation through a coupling for interlocking the diaphragm control mechanisms respectively equiped in the interchangeable lens and in the camera body. Thereby the diaphragm aperture may be changed from the full to the smallest size until it is stopped or arrested by an electromagnetic means in response to an output from an exposure control circuit which compares set exposure factors with the scene brightness measured through the diaphragm aperture being changed as mentioned above. The exposure control circuit generates the output upon detecting that the diaphragm aperture, at the time when scene brightness is being measured therethrough, is proper for the scene brightness and set exposure factors, or when the output of the light measuring circuit of the exposure control circuit, which detects scene light through the diaphragm aperture, becomes proper for the set exposure factors.
This diaphragm control system is favorable in that diaphragms of various interchangeable lenses may be controlled by a single mechanism provided in the camera body, through the coupling which interlocks the diaphragm control mechanisms respectively equiped in the camera body and in an interchangeable lens mounted on the camera, for the purpose of stopping-down the diaphragm from its fully open viewing position to a photographing position of a desired aperture. However, as such coupling usually has and can have no braking mechanism, it is expected that the diaphragm and/or diaphragm control mechanism bounces and oscillates for some time after they are stopped suddenly, while being stopped-down, by the electromagnetic means. Hence, the reflex mirror of the camera should be driven from a viewing to a photographing position some time after the stopping of the diaphragm, when the diaphragm has become stable. This time is referred to hereinafter as release lag.
If the diaphragm control system is designed to provide long release lag for reducing such bounding, there results a long time lag from the shutter release operation to the actual operation of the shutter, and it is difficult to take pictures at the desired instants, especially in the case of moving objects. However, if the system is so designed to drive the diaphragm very rapidly, there results an increase of the bounce and osciliation of the diaphragm and/or its control mechanism, so that precise control of the diaphragm can not be expected.
Further, if the system is so designed that the reflex mirror is actuated at a considerably early time while the diaphragm is stopped-down at proper speed, then it is likely that the mirror is actuated before the diaphragm aperture is determined, especially in the case where the diaphragm must be stopped-down to a substantial extent.
Accordingly, it is necessary for such a system to be designed such that the release lag is short enough but not so short as to affect the precision of the diaphragm control, and so that the release lag is constant irrespective of the diaphragm aperture to be determined.
To comply with this requirement, a device has been proposed which is provided with another electromagnet for driving the reflex mirror at the proper instant as described, for example, in Japanese patent application No. 50-99731 laid open on Aug. 7, 1975. However, this proposed device is unfavorable in requiring further consumption of electric power for the additional electromagnet and control circuit therefor. As the capacity of battery ordinarily adapted for the camera is very limited, such increase of electric power consumption is undesirable. Further, due to the additional electromagnet, the camera will be more expensive and bulky. Still another disadvantage of this camera is that the camera is inoperable when the battery is exhausted.